What sizes and shapes say about seashells

Fresh raw Clam close up on white
Orlando Bellini/Fotolia

Clam shells, like trees, have rings that mark the passage of time. The size and shape of those rings also offer a road map of climate conditions during those passing years.

One new study of shell rings, for instance, suggests that humans have reversed the way the ocean and the atmosphere naturally interact.

“The planet is covered in 70 percent ocean. We wanted to be able to understand weather and climate and what happened before people really started to alter things,” said Al Wanamaker, associate professor at Iowa State University and part of the study.


Researchers analyzed ocean quahogs to understand their chemical makeup. Each ring equals one calendar year, and oxygen isotopes found in the rings tell scientists about ocean conditions. “Embedded in the seashell itself is an archive of ocean conditions through time,” Wanamaker said.

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When clams suck in seawater to eat microscopic plants, they also use the water to create calcium carbonate, a mineral that binds new layers of the shell together. It’s the same mineral that crusts sink faucets and stains them white.

To create a continuous record of yearly sea temperatures, scientists recorded the chemical composition of rings on both dead and living clam shells. They ended up with annual data that span 1,200 years — the longest record yet.

“To have a data point on this, every year for [about] a thousand years, is very unique,” said Valerie Trouet, associate professor of the University of Arizona’s Laboratory of Tree-Ring Research. “If we better understand how the ocean-atmosphere system works, we can develop better models for where the future will bring us.”

That future has plenty at stake: According to the data, oceans led the atmosphere’s temperatures for the first 800 years, but the industrial revolution in the 1800s started to reverse that process. Continuous output of greenhouse gases in the atmosphere leads the ocean to absorb more heat. That means coral bleached white by warming oceans — which robs fish of their homes and their camouflage — on top of other consequences.


In other words, entire ecosystems that have lasted on earth several millennia are in trouble, and it’s partly because humans may have accidentally tipped the scale toward atmospheric dominance.

Kelly Kasulis is a journalist living in Boston and the deputy digital editor of The GroundTruth Project. Follow her on Twitter @KasulisK.